An optical signal propagating in optical fibers, in communication nodes or in other optical telecommunication devices in which spontaneous noise is present, inevitably suffers optical losses and is modified. The signal must be regenerated to compensate the noise accumulated during propagation of the signal and distortion and time shifting of the signal.
To be more specific, the invention proposes to use an optical regenerator to regenerate a multiplexed signal on all the WDM channels in parallel.
The objective that the invention seeks to achieve is therefore to provide a regenerator which has only one optical input and only one optical output. A WDM signal is fed into the single input and all of the regenerated WDM channels are recovered at the single output. Regenerating the WDM channels implies cleansing them of noise, which is in fact to a considerable degree tied to the propagation of light over long distances. In the case of WDM optical signals, the power of the signal on each channel is modulated.
The signal on a channel with a given wavelength therefore comprises pulses representing high logic levels (“1”) and low logic levels (“0”).
Accordingly, once noise at the “0” levels or noise between the “1” levels has been eliminated, an increase in the propagation distance can be expected, because the noise has been removed, and signal shaping and synchronization are virtually automatic.
A regenerator of the kind referred to above is disclosed in French Patent Application No. 98/12430 filed Oct. 5, 1998, referred to hereinafter as document D1, whose title in translation is “A device including a saturable absorber for regenerating a WDM signal”.
In the above patent document, the WDM optical signal to be regenerated comes from an optical fiber and is intended to be injected back into the same optical fiber or into another fiber. The device described includes a dispersive medium, which receives the WDM signal and emits a corresponding dispersed wave into a free space, and a saturable absorber, which receives the dispersed wave and transmits a corresponding regenerated wave.
A saturable light absorber is an optical device consisting in particular of a material which absorbs a low-power optical signal but is transparent to signals of high light power. Thus the material of a saturable absorber is increasingly transparent to a light beam as the power of the beam increases.
As already pointed out, in the case of WDM optical signals, the power of the signal on each channel is modulated. When a saturable absorber receives a high power optical pulse it becomes transparent and allows the pulse to pass through it. On the other hand, the saturable absorber becomes absorbent for lower power noise between the pulses, and attenuates the noise.
Thus, according to the teaching of the prior art, the wave from an optical fiber is focused onto a saturable absorber strip at points that differ according to the wavelengths of the WDM channels because of the dispersive medium of the device.
FIG. 1 shows in section a prior art saturable absorber strip that receives a dispersed wave and transmits a corresponding regenerated wave. The active layer 2 of the absorber 1 is conventionally made from a ternary material, for example InGaAs or AlGaAs, and includes multiple quantum wells. It could equally well be made of a quaternary material. Two reflectors 3 and 4 are placed parallel to and on opposite sides of the active layer 2 to cause multiple reflections of the light wave passing through the active layer 2.
Because of the multiple reflections, the light wave passes through and is absorbed by the active layer 2 several times, which has the advantage of reducing the required thickness of the active layer. The bottom reflector 4 is deposited on a layer 5 forming the substrate, for example an InP layer.
The combination of the active layer 2, the top reflector 3, and the bottom reflector 4 has a uniform thickness e over the whole of the length of the saturable absorber strip 1.
Using the above kind of saturable absorber strip to regenerate a WDM signal is known in the art. FIG. 2 shows a prior art WDM signal regenerator described in document D1 which uses the strip shown in FIG. 1.
The optical signal from the fiber A is projected by a lens B1 onto a grating B2.
The grating B2 separates the optical signal into a plurality of light beams having different wavelengths and deflects each light beam at an angle that depends on the dispersion coefficient of the grating and on the wavelength of said beam. A second lens B3, situated at the exit from the grating B2, then focuses each beam deflected by the grating onto a spot on the saturable absorber C.
The spot associated with each light beam is in fact focused on the absorber C at a location x which depends on the dispersion coefficient of the grating B2 and on the wavelength of the deflected beam.
Thus each wavelength of the WDM optical signal corresponds to a respective point where its spot is focused on the saturable absorber C.
After regeneration in the saturable absorber, each of the beams must be returned the way it came in order to be redirected towards the input fiber A, after recombination of all the regenerated beams by the grating B2.
To be able to reflect the regenerated signal, the saturable absorber strip C has a reflective coating or a Bragg reflector on its second face C1, which is generally perpendicular to the direction of propagation of the wave that it receives.
A spatial separator, for example a circulator F, must be provided at the other end of the fiber A. The circulator F separates the optical signal to be regenerated, which travels in one direction, from the regenerated optical signal, which travels in the opposite direction.
However, the prior art saturable absorber structure used in the FIG. 2 optical regenerator does not provide the ideal processing to regenerate a WDM signal.
Thus problems are encountered with the prior art saturable absorber structure as described, and are unacceptable if correct regeneration of a WDM signal is the aim.
In particular, a first drawback of the prior art regenerator is the difficulty of recovering the regenerated signal.
Each of the beams corresponding to a particular wavelength channel of the WDM signal is focused onto the saturable absorber with a different angle of incidence relative to the optical axis of the lens B3.
The front mirror of the saturable absorber is typically adjusted to reflect beams impinging on its surface perpendicularly. Because of the different angles of incidence, reflection at the front mirror of the saturable absorber of each of the beams corresponding to respective wavelength channels of the WDM signal cannot be controlled accurately, which makes it impossible to recover the complete regenerated WDM signal in the fiber A.
Also, not all wavelengths can resonate at the same time. For the wave associated with a light beam impinging on the saturable absorber strip to be regenerated effectively, the amplitude peak of the wave, i.e. the anti-node of the wave, must be at the level of the active layer of the saturable absorber, to maximize interaction between the wave and the active layer of the absorber.
The condition for resonance is given by the following equation, which establishes the correspondence between the incident wavelength and the thickness of the saturable absorber cavity:λ=neff·e/k
where:
λ is the wavelength of the wave crossing the saturable absorber cavity,
e is the thickness of the saturable absorber cavity,
neff is the effective index of the medium, and
k is an integer and indicates the order of resonance.
As the prior art strip has constant thickness e throughout its length, the adjustment that satisfies the condition of resonance is obtained for a very specific wavelength satisfying the above condition. In the case of a WDM signal comprising 25 different wavelengths λ1 to λ25, for example, the condition for resonance cannot be satisfied at all of the wavelengths. Thus, if the saturable absorber strip is adjusted for the value λ1, on moving away from this adjustment, the thickness e of the strip being constant, the condition for resonance will no longer be satisfied at λ25. In other words, it is not permissible to obtain the wave associated with the resonant wavelength λi in the active layer regardless of the value of i, from 1 to 25 in this example. The regeneration of the WDM signal is therefore less than the optimum, because the condition for resonance is not satisfied at all wavelengths. In fact, some channels are given preference over others.
What is more, considering a reference position corresponding to that of the light beam which impinges on the front face of the saturable absorber strip perpendicularly, it can be shown that as the distance from this reference position increases, so does the angle of incidence of each light beam associated with a WDM channel of wavelength λi. Physically, the lens B3 focuses each light beam on a spot which occupies an area of the saturable absorber.
After multiple reflections of the wave within the active layer of the absorber, as described with reference to FIG. 1, this area is offset for beams with a large angle of incidence.
In this case, the effective area which corresponds to the overlap within the active layer between the area occupied by the spot before regeneration and the area occupied by the spot after regeneration is therefore considerably reduced. There is therefore a problem of overlapping of the area occupied by the spot when the angle of incidence is high.
Finally, a last prior art problem concerns crosstalk. To each wavelength constituting the WDM signal there corresponds a point of focusing of the associated spot on the saturable absorber, and these focusing points are very close together. Crosstalk then arises as charge carriers diffuse in the material forming the active absorber layer of the strip when the latter is illuminated at a focusing point.
If the focusing points are too close together, the diffusion of these charge carriers at one focusing point interferes with the portion of the absorber material adjoining it and the processing (noise absorption) of the corresponding WDM channel will therefore be defective.